1. Field of the Invention
The present invention relates to a welding shield gas used in welding stainless steel, in particular, austenitic stainless steel, and in particular, to a welding shield gas appropriate for non-consumable electrode arc welding in which the precision of the welding is important, and relates to a welding method using the same.
Priority is claimed on Japanese Patent Application No. 2003-096770, filed Mar. 31, 2003, the content of which is incorporated herein by reference.
2. Description of Related Art
Generally, stainless steel tubing is used as tubing for supplying a gas used in semiconductor fabrication and the like to the fabricating apparatus.
In the welding of tubes, a non-consumable electrode arc welding, for example, I-type butt welding using automatic TIG (tungsten inert gas) welding without using a filler material, is widely used. As a shield gas, a mixed gas consisting of Ar and several percent H2 or pure Ar can be used.
The weld zone of the tube preferably does not have irregularities on the surface thereof. In particular, it is important that at the inner wall surface of the tube at the weld zone, the penetration bead of the weld zone does not form irregularities with respect to the inner wall surface side of the tube, and furthermore, the bead width must fall within a suitable range.
For example, in the I-type butt welding, when the heat input during welding becomes excessive, the amount of melting becomes excessive, and the weld zone considerably widens. In the case of flat position welding, due to the influence of gravity and the like, the bead penetration forms a protrusion with respect to the inner wall surface. In this case, fine metal particles called fume are ejected into the tube, and thereby contamination of the supplied gas due to these particles easily occurs.
In addition, both weld metal having low corrosion resistance and the heat affected zone in the weld zone are present, and thus when the amount of melting is excessive, there is the problem that corrosion occurs over a wide area.
In addition, when heat input into the weld is insufficient, the stability of the arc deteriorates due to consumption of the electrode, or the weld is mispositioned due to human error, the abutted parts are not completely melted up to the inner wall surface of the tube and thus irregularities form due to clearances between the abutted parts of the tubes. In this case, a concavity that forms becomes dead space, and dust particles may accumulate therein. Thus, there is the problem that these accumulated particles are carried along with the supplied gas to cause contamination.
Furthermore, there are the problems that air present in a concavity makes gas exchange difficult and contaminates the supplied gas with impurities.
In addition, when supplied gas is a corrosive gas that is used, for example, in semiconductor fabrication, there are the problems that crevice corrosion in a concavity is exacerbated, the metal corrosion products generated due to the crevice corrosion peel off, and thereby the supplied gas becomes contaminated. Furthermore, due to a concavity, there is the problem that the mechanical strength of the tube deteriorates.
Thus, a method for inspecting the weld zone has been proposed that has as an object confirming whether or not the welding has been properly carried out (refer to Citation 1: Japanese Unexamined Patent Application, First Publication No. 2001-074712).
It is possible to improve the reliability of the tube if the weld zone is inspected, but because the inspection is not a complete solution to this problem, a method that prevents the welding problems themselves is desired.
In addition to insufficient heat input into the weld, the deterioration in the stability of the arc due to consumption of the electrode, or the mispositioning of the weld is due to human error, one factor that hinders the complete melting of the abutted parts is the deterioration of the bead progress caused by slag that forms on the surface of the weld pool during welding.
When the bead progress deteriorates, even if the targeted position of the weld is accurately aligned with the abutted parts, there are cases in which the center position of the bead becomes separated from the abutted parts, and the wall surface in the tubes of the abutted parts are not completely melted.
The deterioration of the bead progress occurs easily when the diameter of the tube is not less than 20 mm and the thickness is not less than 1.2 mm and equal to or less than 3 mm. In addition, when the welding current used is in a low current region of not more than 100A, bead progress deteriorates easily.
It is known that the arc easily becomes unstable when the welding current is low, and to improve the stability of the arc, a pulsed welding method, in which the welding current is periodically set to a high value, is effective. By using pulse welding, the bead progress can be improved to some extent.
However, completely preventing the welding problems by using this method has been difficult.
In addition, when the weld current is pulsed, there is a tendency for the bead face width to become wide in comparison with the penetration bead width, and there are the problems that a weld zone is formed larger than is necessary, the method of setting the welding conditions becomes complicated, and residual defects in the abutted parts that cannot be discovered by visual inspection of the outer surface of the tube occur easily.
As a measure that can be taken against the deterioration of the bead progress, increasing the welding speed and increasing the weld current can be considered. However, when the welding speed and the weld current are increased, the weld penetration deteriorates, easily causing, for example, undercut and an increase in the amount of concavity.
In addition, if the groove shape has a single-V groove or a single-U groove and the root surface height is made as small as possible, residual defects in the abutting parts occur with difficulty. However, in this case, because a costly automatic welding machine that can add a filler material becomes necessary, there is a drawback in terms of cost, and the welding method becomes complicated. In addition, there is the problem that contamination inside the tube occurs easily during the groove preparation due to oil, particles and the like.
Widening the bead width such that the abutted parts can be welded even when progress characteristics deteriorate or using manual welding can also be considered as measures to be taken against bead progress deterioration.
However, there are the problems that widening the bead width invites deterioration in the weld quality and using manual welding requires much labor, which in turn causes a lowering in the welding efficiency.
There is also the possibility that slag generated in the weld pool may contribute to the deterioration in the bead progress. Using a tube consisting of a material having a reduced slag component content can be considered to prevent slag generation (refer, for example, to Japanese Unexamined Patent Applications, First Publication Nos. Hei 7-197203 and 2002-220618).
However, when using a tube consisting of this material, a sufficient effect is not obtained under the welding conditions or tube shape described above.
As described above, completely preventing welding problems is difficult using already existing technology.
In consideration of the problems described above, it is an object of the present invention to provide a welding method that does not cause a deterioration in the welding efficiency and prevents the occurrence of welding problems at low cost when welding stainless steel, and a welding shield gas that can be used with the same.